Relays increasingly play a role in increasing range and/or signal quality in wireless communication systems. For example, Release 10 of the Long Term Evolution (LTE) standards includes support for the use of relays to improve cell-edge coverage and capacity in a cost-effective manner.
In an example relaying scenario for the LTE case, a donor base station in the LTE network—referred to as a donor eNodeB (DeNB)—connects to a relay node that in turn connects to user equipments (UEs). The DeNB transmits downlink data to the relay using an LTE access link and in turn the relay forwards the data to targeted UEs that are supported by the relay. Correspondingly, when one of the UEs has uplink (UL) data available, it transmits that data to the relay node and the relay node transmits it to the DeNB. The LTE standard refers to the link between the DeNB and the relay node as the Un or backhaul link and refers to the link between the relay node and the UEs as the Uu or access link.
The backhaul link between the DeNB and the relay node is similar to the traditional link between the eNB and UE. The link implements normal LTE L2 protocols such as PDCP (Packet Data Convergence Protocol), RLC (Radio Link Control), HARQ (Hybrid Automatic Repeat reQuest), and RRC (Radio Resource Control). Thus, the behavior of the relay node towards the DeNB in this link is similar to the behavior of a UE. Scheduling of radio resources on the backhaul link is controlled by the donor eNB. Thus, uplink scheduling in the backhaul link reuses the scheduling mechanisms standardized for UEs.
To understand these scheduling mechanisms, first consider the nominal UE-to-eNB scheduling case. According to the standard UL scheduling procedure defined for LTE Rel-8/Rel-9 (see 3GPP TS 36.201), a UE triggers a Buffer Status Report (BSR) when new data arrives in its empty transmit UL buffer, or when the newly arrived data belongs to a higher priority logical channel group than any existing data in its UL transmit buffer. If the UE does not have UL resources to transmit the BSR, it triggers a Scheduling Request (SR) to the eNB.
The eNB schedules the UE responsive to receiving the SR and the UE uses the initial grant to transmit the BSR and potentially also other data up to the size of the grant. The BSR indicates to the eNB how much remaining UL data the UE has to transmit and the eNB uses that information as an input to its UL scheduler. For example, the UL scheduler in the eNB uses the information to determine the size and number of further grants to schedule, for transmission by the UE of its remaining UL data.
For many applications, short access delays play a crucial role in providing good end user performance. As one example, International Mobile Telecommunications Advanced (IMT Advanced) latency requirements state that the one way radio access delay between the mobile terminal and the base station should be under 10 ms. Latency concerns arise if the standard UL scheduling procedure is applied to the relay-node-to-DeNB link as well as to the UE-to-relay-node link—i.e., on both the Uu and Un interfaces.
See FIG. 1 for an illustration of the above “standard” case in the context of Uu and Un links. In the figure, data arrives in the UL buffer of a UE connected to a relay node and in response the UE transmits an SR to the relay node. With some processing delay, the relay node schedules the UE with a small initial grant. With the received grant, the UE transmits a BSR on the Uu link to the relay node, potentially with some UL data. The UE's UL data received by the relay node represents UL data to be sent from the relay node to the DeNB on the Un link.
Thus, in response to receiving any UE UL data, the relay node transmits an SR on the Un link to the DeNB. After some processing delay at the DeNB, the relay node receives a return grant from the DeNB, and the relay node sends another BSR to the DeNB using the granted resources. The relay node BSR includes information regarding UL data buffered in the relay, including data received from the UE.
From this simple illustration, one sees that the user plane delay over the Uu and Un links is potentially significant—e.g., about 20 ms from transmission of the SR from the UE to transmission of the BSR from the relay node. This size delay may be too much for certain applications. In an attempt to at least partially address the delay problem, contribution “R2-102252” was presented in RAN #69bis.
The '252 contribution proposed certain mechanisms to reduce latency in the user plane, for relay-supported links. For example, it proposed that a relay node triggers a BSR procedure right after receiving a BSR from UE, where the relay node BSR includes the UE BSR. While this mechanism offers delay reduction in certain scenarios, it does not adequately address initial delays in cases where small amounts of data are transmitted, nor does it necessarily provide the DeNB with timely information when data is available in the UL buffer at the relay node.